Protein kinases play an important role in a large percentage of the biochemical processes that regulate the functions of cells that are critical in tumor developments including; cell proliferation, genomic repair, apoptosis, migration and invasion. These proteins serve, in many cases, as molecular “switches” regulating the activity of target proteins through the process of phosphorylation. In normal cell physiology, the coordination of multiple kinases is a tightly regulated process allowing the cell to function in a manner in which it was designed. Protein kinases and phosphatases play a prominent role in the tumorigenic process. Normal cell physiology is dependent on the appropriate balance between kinase and phosphatase activity to keep important signaling pathways within tolerated levels. Mutations in the genes that encode these proteins often leads to aberrant signaling that lays the foundation for changes in cellular function. Alterations in numerous protein kinase pathways ultimately lead to deregulation of cellular function that affect pathways that are hallmarks of the tumor phenotype.
One kinase pathway that plays a prominent role in tumor development and progression is the phosphoinositol 3 Kinase (PI3K)/Akt pathway. This pathway typifies the multi-component regulatory mechanisms that regulate normal cell function but lead to malignant phenotypes when proteins are genetically modified and aberrantly regulated. Many of the proteins in this pathway are genetically altered and aberrantly activated conferring tumorigenic properties in cultured cells and in human tumors (e.g. see A. Carnero, Curr Pharm Des, 2010, 16:34). Multiple kinases in this pathway have been the subject of pharmacological intervention. One kinase in this pathway, the phosphoinositide-dependent kinase1 (PDK1), is a critical activator of multiple proteins involved in pro-survival and oncogenic activity. As such, it provides drug development groups an attractive target for new cancer therapies.
Activation of PI3K by engagement of cell surface receptor tyrosine kinases by insulin and growth factors generates phosphatidyl-inositol,3,4,5 triphosphate PIP3 (2). PDK1 and Akt are recruited to the cell membrane and subsequently activated in response to increases in PIP3 generated by the activity of PI3K. The recruitment of PDK1 and Akt to the cell membrane is mediated through interactions of homologous pleckstrin homology domains. Localization of these proteins to the plasma membrane allows PDK1 to activate AKT by phosphorylation at residue threonine-308 (e.g. see L. Stephens et al., Science 1998, 279:710). Activated PDK1 phosphorylates Akt as part of an important signaling pathway that ultimately regulates the signaling of multiple biological processes. As a transducer of the PI3K signal and as a regulator of numerous kinases involved in promoting cancer growth, proliferation and survival, PDK1 distinguishes itself as an attractive target for drug development.
It has also been observed that about 50% of common human tumor types possess mutations in genes that regulate PIP3 production, and these mutations impart these cancer cells with abnormally high levels of this second messenger (Vanhaesebroeck, B., et al. Ann. Rev. Biochem., 70:535-602 (2001)). A common mutation affecting PIP3 production is in PTEN, the lipid phosphatase that breaks down PIP3. The finding that mice expressing half the normal amount of PTEN are protected from developing a wide range of tumors by reducing PDK1 expression levels supports this idea. The potential of PDK1 inhibitors as anti-cancer compounds has also been suggested by transfection of a PTEN negative human cancer cell line (U87MG) with antisense oligonucleotides directed against PDK1. The resulting decrease in PDK1 protein levels led to a reduction in cellular proliferation and survival (Flynn, P., et al., Curr. Biol., 10: 1439-1442 (2000)). The PDK1/Akt pathway is activated in many cancer via mutations in other proteins such as Receptor Tyrosine Kinases (RTKs), Ras, or PI-3 kinase (Cully et al., Nature Reviews Cancer 6:184-192 (2006)). Mutations in PDK1 itself have been found to be associated with a variety of cancer types. For example, the identification of PDK1 mutations (PDK1 T35414, PDK1 D527E) in human colorectal cancers suggests that inhibitors of this kinase may have therapeutic value by directly inhibiting either wild-type or mutant forms of this protein. See, Parsons et al., Nature 436, 792 (11 Aug. 2005).
In summary, PDK1 is a central activator of several signaling pathways that are frequently altered in human cancers making it an attractive target for therapeutic intervention. Consequently, there is a great need in the art for effective inhibitors of PDK1.